The invention relates to a method for reliably cooling turbines such as air storage turbines and, in particular, for the period while the rotors run down after termination of the power operation.
After the termination of the power operation of an air storage gas turbine by closing the valves for the supply of the working medium, the turbine rotor continues to rotate for a certain period. During this turbine run-down, also referred to as transient windage operation, a residue of the working medium is still present in the turbine. The dissipation of energy to the blading can then lead to a generation of heat which heats the turbine to an unallowable extent. For this reason, the blading and other turbine parts are cooled with air in order to avoid overheating of the turbine and ensure risk-free, transient windage operation. Such cooling is particularly important for those components which are located within the region of influence of blading with a length which is large in relation to the radius of the turbine rotor.
In known air storage gas turbines, the cooling air to the turbine is supplied as a function of the time after the closing of the turbine valves, in accordance with a method shown in FIG. 1. The full curve, which is also designated by (m/m0)S, shows the relative mass flow of the cooling air supplied, which mass flow is the consequence of a typical time-cycle valve control. The supply of cooling air only begins approximately 5 seconds after the rapid closure of the turbine valves and remains, as compared with the supply with a fully open valve, almost unaltered and only falls very slowly during a long phase. It then falls rapidly.
In such installations, it has been found that, during the first phase, the cooling air flow is located far above the actual requirement for cooling air, such as would be actually necessary for the removal of the heat generated. Because of this, the turbine components are cooled to temperatures which are far below the maximum permitted limiting temperature. This leads to higher transient stresses in the turbine components and these can reduce their life. In addition, the overdimensioned cooling air flow signifies an unnecessary loss of storage energy.
FIG. 2 shows, as a full curve, the variation of the relative flow coefficient xcexd/xcexd0, xcexd being the ratio of the axial flow velocity to the peripheral velocity of the blades. The full curve (xcexd/xcexd0)S represents the flow coefficient which occurs as a consequence of a cooling air supply according to the prior art. In a quasi steady-state range between t1 and t2, in which the differential coefficient of (xcexd/xcexd0)S tends to 0, critical stress amplitudes, which can cause damage, occur in the blading. The shaded region B corresponds to the region B shown in FIG. 3.
FIG. 3 shows the relative stress amplitude "sgr"/"sgr"0 of a typically affected long blade/vane for quasi steady-state conditions, or more precisely as a function of the relative flow coefficient (xcexd/xcexd0). Whereas, under quasi steady-state conditions, this parameter (xcexd/xcexd0) is reduced, it first passes through a phase D of low stress amplitudes, such as arise during normal turbine operation. After this, the stress amplitudes increase slowly during a transition phase C. During a next phase B, the stress amplitudes of the large blades/vanes attain critical levels and, after this, they fall rapidly in the last phase or windage phase A. During the run-down, it is therefore important to pass through the phase B with the critical stress amplitudes as rapidly as possible in order to avoid inter alia dangerous blading vibrations and possible blading damage.
Finally, in the case of a different rotational speed function, such as, for example, without overspeed or larger or smaller speed gradients, inappropriately high or low temperatures occur with this time-dependent supply of cooling air.
The object of the present invention is to create a safe and protective cooling method for an air storage gas turbine, which method is employed inter alia during the run-down or idling of the turbine after closure of the supply valves for the working medium of the turbine. The cooling method is, in particular, intended to avoid thermal stresses in the components of the turbine due to non-optimum cooling and high stress amplitudes resulting from blading aerofoil vibrations.
This object is achieved by means of a method according to claim 1.
In the turbine cooling method according to the invention, the mass flow of a cooling medium is turned down, after shut-off of the supply of the turbine working medium, in proportion to a third power of the peripheral velocity of the blades or the third power of the rotational speed of the rotor. This control of the supply of the cooling medium reduces the mass flow during the run-down of the turbine to suit the rotor speed and reduces it to a sufficient extent, as compared with the method mentioned of the prior art. Due to the control, according to the invention, of the cooling air supply, only as much heat is removed as occurs due to windage. This ensures that the temperature of the critical turbine components remains within an acceptable range, i.e. the components do not overheat but are not excessively cooled either. No significant, transient thermal stresses in the components occur either, therefore, because undercooling or overcooling are avoided.
For a certain peripheral velocity of the blades, the reduced mass flow of the cooling medium ensures that the relative stress amplitudes of the long blades are minimized and remain at a level under the maximum blade vibration amplitude, by which means the risk of blade damage is completely checked. In effect, the cooling method according to the invention has the result that during the reduction of the mass flow through the turbine, the critical phase B shown in FIGS. 2 and 3 is only passed through once, and is passed through rapidly, and in consequence intolerably high stress amplitudes cannot appear. After the windage phase A has been reached, the critical phase B is no longer attained because of the reduced mass flow of the cooling medium; in consequence, the major part of the run-down of the turbine takes place in the safe and risk-free windage region A.
In a preferred cooling method, the supply of the cooling medium is realized with a time delay after the supply of the working medium has been shut off. This provides the advantage that any turbine drive is avoided, particularly in the case of a rapid shut-down with overspeed.
In a further preferred cooling method, the supply of the cooling medium is started as soon as the differential coefficient with respect to time of the rotational speed has a negative value. In the case of overspeed, this method avoids any overtemperatures in the turbine.
In a preferred cooling method, the cooling medium is admitted via the turbine inlet and/or via the cooling systems of the rotor and the guide vanes. A supply via the blading duct and/or via the cooling systems for the rotor and the blading permits sufficient cooling of the last two turbine blading rows and the adjacent components such as, for example, the diffuser duct, blading supports and rotor. The cooling of the last stage is, in particular, ensured because the temperature difference between the cooling medium and the last blading rows, which have to be cooled, is still sufficient for an adequate cooling effect. The method also ensures that all the critical components, in particular the rotor and the last two stages, are sufficiently cooled.
In the case where the cooling method is applied to an air storage gas turbine, the cooling medium is introduced before the low-pressure turbine, preferably via the cooling systems of the rotor and the guide vanes. Tempered air, preheated to an adequate temperature, is preferably used for cooling. This can, for example, be preheated in a recuperator. In the case of an air storage power installation, a reduction in the mass flow of the cooling air also minimizes the loss of stored energy.
The cooling method according to the invention can also be employed in the case of steam turbines, in particular in the case of a vacuum failure and accelerated run-down of the steam turbine.
Because the cooling method according to the invention is independent of time, it also ensures sufficient cooling in the case of faults such as delayed or accelerated run-down of the turbine for which, otherwise, no sufficient cooling air supply could occur.